A number of peptides originally characterized in non-neural peripheral tissues have also been reported to be stored and presumably released with the posterior pituitary hormone oxytocin (OT). These co-localized peptides (methionine enkephalin, cholecystokinin, gastrin and renin) may modulate OT release. Alternatively, preliminary investigations in my laboratory suggest that these peptides may also modulate OT-induced uterine contraction. A specific aim of the proposed research is to determine the significance of co-localized peptides during labor. To accomplish this aim, the effects of the co-localized peptides on OT-induced contraction of the diethylstibesterol-treated rat uterus, in vitro, will be determined. To find if the pituitary content of co-localized peptides varies during states in which OT is physiologically important, the peptide content of pituitary extracts from rats during pregnancy, labor and the post-partum period will be determined by radioimmunoassay. These two studies will provide information regarding the importance of co-localized peptide effects on labor. Effects of co-localized peptides on the potency and efficacy of OT will be assessed by comparisons of the outcome of labor, in rats, induced by OT alone or by OT plus co-localized peptides. This knowledge could provide information which would allow more effective use of OT to induce or augment labor in women. The long term objective of the proposed research is to provide information about the physiology of normal labor which might suggest a basis for development of drugs to modify uterine contractions during labor.